iov_iter: Export import_ubuf()

Export import_ubuf() to be used in sound subsystem for generic memory
handling as Linus suggested.  It's used for constructing an iov_iter
of a single segment user-space copy for PCM data.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/CAHk-=wh-mUL6mp4chAc6E_UjwpPLyCPRCJK+iB4ZMD2BqjwGHA@mail.gmail.com
Link: https://lore.kernel.org/r/20230815190136.8987-2-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>